Patent Literatures 1 and 2 discuss a projector that uses a LED (Light Emitting Diode) as a light source for illuminating a DMD (Digital Micromirror Device) serving as a display element. The projector of this type is expected to reduce costs more than a projector that uses a discharge lamp, and achieve performance including miniaturization, lower power consumption, and a wider color reproduction range.
There are also a projector that uses a single LCoS (Liquid Crystal on Silicon) element (Nonpatent Literature 1), and a projector that users three liquid crystal panels as display elements (Nonpatent Literatures 2 and 3).
In the liquid crystal projector, usually, a liquid crystal panel must be illuminated with polarized light. Generally, light from the LED is unpolarized light. Thus, when the LED is used as a light source for illuminating the liquid crystal panel, it is preferable that the light from the LED be converted into polarized light to increase light use efficiency
For example, when a display panel is a TN liquid crystal (Twisted Nematic Liquid Crystal) panel, with respect to the unpolarized light from the LED which includes two linear polarized components which are orthogonal each other, one of linear polarized component is converted to be similar to the other polarized component. When efficiency of this polarization conversion is low, the light use efficiency declines. When no polarization conversion is carried out, about a half of the light is not used as illumination light.
In the projector described in Nonpatent Literature 1, a luminous flux from the LED that is the light source is roughly converted into parallel luminous fluxes by a light guiding member having a composite parabolic surface shape, which is referred to as a CPC reflector. The display panel is illuminated with the parallel luminous fluxes from the light guiding member.
A quarter-wave plate and a reflective polarization plate are located in the traveling direction of the luminous flux from the light guiding member. For example, the polarization plate transmits p-polarized light while reflecting s-polarized light. The s-polarized light reflected by the polarization plate passes through the quarter-wave plate. The light (circular polarized light) passed through the quarter-wave plate returns to the LED to be reflected on a surface of the LED. The light reflected on the surface of the LED is converted parallel by the light guiding member, and then passes through the quarter-wave plate again.
A portion of the s-polarized light reflected by the polarization plate, which has passed through the quarter-wave plate twice in the returning process to the LED and in the process of being reflected on the surface of the LED to travel toward the polarization plate, is converted into p-polarized light.
In addition, there is an illuminating device discussed in Patent Literature 3. This illuminating device includes: a light source; a glass rod one end surface of which light from the light source enters; condensing means for condensing a luminous flux that has exited from the other end surface of the glass rod; and a PBS polarization conversion array disposed at a position where the condensing means forms a plurality of light source images.
The PBS polarization conversion array has first and second prisms that are arranged alternately in one direction. Each of the first and second prisms is a rectangular parallelepiped-shaped prism composed of two right-angled prisms that are adhered together.
The first prism includes a polarizing separation film formed on the bonded surface of the two right-angled prisms to transmit the p-polarized light while reflecting the s-polarized light, and is configured so that incident light can enter the polarizing separation film at an incident angle of about 45 degrees.
The second prism includes a reflection film formed on the bonded surface of the two right-angled prisms, and is configured so that the s-polarized light reflected by the polarizing separation film of the first prism can enter the reflection film at an incident angle of about 45 degrees. A surface of the second prism located in the traveling direction of the light reflected by the reflection film is an exit surface. A half-wave plate for converting the s-polarized light into p-polarized light is formed on the exit surface. A light shielding plate is formed on a surface opposite the exit surface of the second prism.
P polarized light that exits from the first prism travels in the same direction as does P polarized light that exits from the second prism. The condensing means is structured such that an image of the light source is formed on the incident plane of the first prism.